1. Technical Field
The present invention relates generally to a threshold detecting method which performs frequency analysis (e.g., an FFT algorithm) on a beat signal, as produced in a FMCW (Frequency Modulated Continuous Wave) radar, and determines a threshold value for use in detecting a peak frequency component arising from an echo of a transmitted radar wave from a target object, an object information producing apparatus designed to use the threshold detecting method to produce information about a target object tracked by a radar, and a computer readable program including instructions to perform the threshold detecting method.
2. Background Art
Japanese Patent First Publication No. 11-271431 discloses an FMCW radar system which frequency-analyzes a beat signal, as produced by mixing a transmitted signal and an echo thereof from a target object, and detects or extracts a peak frequency component which has arisen from the echo from the target object from results of the frequency analysis using a peak detecting threshold. This publication also teaches a technique of determining the peak detecting threshold based on a frequency spectrum created by removing the peak frequency component representing the target object from frequency components derived by frequency-analyzing the previous beat signal.
Japanese Patent First Publication No. 2001-91642 teaches a technique of calculating a moving average of a plurality of frequency components around a specified frequency component (i.e., a specified frequency BIN) to determine the peak detecting threshold.
Each of the above techniques is to determine the peak detecting threshold using the results of the frequency analysis from which the peak frequency component is required to be extracted, thus resulting in a great change in the peak detecting threshold with a change in environmental condition, which leads to instability in detecting the target object.
Techniques are also known of estimating the thermal noise generated by the radar receiver and calculating the peak detecting threshold based on the thermal noise. Specifically, the level of a signal acquired in the receiver when the receiver is on, but the transmitter is off is defined as a noise level. The peak detecting threshold is determined based on the noise level.
As one source of noises contained in a signal received by the radar, a leakage noise that is noise which leaks from the transmitter and is then added to the received signal duel to a lack in isolation between the transmitter and the receiver. Typical radars are, therefore, designed to have isolation between the transmitter and the receiver so that the leakage noise is buried in a receiver noise that is noise generated by the receiver itself to an extent that it may ignored.
However, in recent years, antennas which are low in transmitter-to-receiver isolation may have been used in development of downsizing the antenna of the radar or reducing the production cost thereof. Use of such a type of antenna may result in generation of leakage noise which is too great in level to be ignored as compared with the receiver noise. Additionally, use of receivers which are lower in receiver noise level for improving the performance of the radar may also cause the level of the leakage noise to be too great to be ignorable relative to the receiver noise.
The above described technique of deactivating the transmitter to measure the noise level has the disadvantage that it is impossible to reflect the effects of the leakage noise on the noise level, thus resulting in a decrease in accuracy in calculating the peak detecting threshold, which leads to inaccuracy in detecting the target object.